At the present time, an organic electroluminescent element is attracting attentions as a thin light-emitting material.
An organic light-emitting element makes use of electroluminescence (EL) of an organic material (hereafter, it is also called as “an organic EL element”). It is a fully solid element which is capable of emitting light with a low voltage such as about several V to several ten V. It has many excellent features of high luminance, high light-emission efficiency, thin and small weight. Accordingly, it has been attracted attentions in recent years used for: various display backlights; a display board such as signboard and emergency lamp; and a surface light-emitting body for illumination source.
The organic light-emitting element has a structure in which a light-emitting layer containing an organic material is located between a pair of electrodes, and emitted light in the light-emitting layer is extracted to the outside through the electrode. Therefore, at least one of the pair of electrode is composed of a transparent electrode, and the emitted light is taken out from the transparent electrode side.
Further, an organic light-emitting element is characterized in doing a thin film surface light-emitting, which is different from the conventional light-emitting body. In order to make use of this feature, it is required to form an element on a flexible transparent substrate. In order to reply such request, it is required to realize an organic light-emitting element employing a widely used resin substrate such as polyethylene terephthalate (PET).
However, when a resin substrate such as polyethylene terephthalate (PET) is simply used for producing an organic light-emitting element, it was revealed that there may occur problem of the electrode which is made of indium tin oxide (SnO2-Inc2O3: ITO), for example. The electrode may be cracked or be floated or be peeled off. In particular, this problem will occur when an internal light extracting layer described later is provided between a transparent substrate and an electrode.
The organic light-emitting element can generate light with high luminance at low electric power, and it is excellent in the points of visibility, response speed, lifetime and electric power consumption. Unfortunately, the use efficiency of the light generated in the organic light-emitting element is at most 20%, which indicates significant loss of the luminescent light inside the element. It is preferable to form an internal light extracting layer for the purpose of increasing light emission efficiency.
FIG. 1 is a schematic sectional view illustrating a constitution of a conventional organic light-emitting element and a propagation way of light.
An organic light-emitting element 100 includes a metal electrode 101, an organic light-emitting layer 102 having a refractive index of about 1.8, a transparent electrode 103 having a refractive index of about 1.8, and a transparent substrate 104 having a refractive index of about 1.6, which are sequentially laminated from a lower layer in the drawing. In the drawing, the arrows 110a to 110e indicate characteristic light components of the light generated in the organic light-emitting layer 102.
The light component 110a is perpendicular to the organic light-emitting layer 102 which is a light-emitting surface, and it is extracted through the transparent substrate 104 from a light extraction side (to the air).
The light component 110b is an incident light on the interface between the transparent substrate 104 and the air at the critical angle or less, and inflects at the interface between the transparent substrate 104 and the air to be extracted from a light extraction side. The light component 110c is an incident light on the interface between the transparent substrate 104 and the air at an angle larger than the critical angle.
The light component 110c is totally reflected at the interface between the transparent substrate 104 and the air, and cannot be extracted from the light-emitting side. This loss due to total reflection of the light is referred to as “substrate loss,” which is typically about 20%.
The light component 110d is an incident light on the interface between the transparent electrode 103 and the transparent substrate 104 at an angle larger than the critical angle and satisfies the resonant condition. Such a light component 110d is totally reflected at the interface between the transparent electrode 103 and the transparent substrate 104 to generate a waveguide mode, in which the light component is confined within the organic light-emitting layer 102 and the transparent electrode 103. This loss due to the waveguide mode is referred to as “waveguide loss,” which is typically about 20 to 25%.
The light component 110e is an incident light on the metal electrode 101, and reacts with free electrons in the metal electrode 101 to generate a plasmon mode, one of the waveguide modes, in which the light component is confined near the surface of the metal electrode 101. This loss due to the plasmon mode is referred to as “plasmon loss,” which is typically about 30 to 40%.
As described above, the conventional organic light-emitting element 100 has substrate loss, waveguide loss, and plasmon loss; hence, light-emitting elements are faced with the task of extracting a larger amount of emitted light by reduced extraction loss.
To cope with this problem, Patent Document No. 1 discloses an organic electroluminescent (EL) device provided with a light scattering unit composed of a lens sheet and disposed adjacent to a light extraction surface side.
Patent Document No. 2 discloses a substrate for a light-emitting device disposed on the light-emitting surface of a light-emitting device, the substrate including an irregular layer having a high refractive index of 1.6 or more and an average surface roughness of 10 nm or more on at least one of surfaces of the substrate and one or more substrate layers having a refractive index of 1.55 or more, and a light-emitting device.
However, although these technologies are preferable embodiments from the viewpoint of light extraction, these are technologies of light extraction for a glass substrate having no flexibility. These technologies have not realized using a substrate having flexibility.
Further, Patent Document 3 discloses a light extracting layer composed of a single or a plurality of laminated layers and used for a light-emitting element. At least one of the constituting layers of the light extracting layer is a layer having a light scattering function, and it is characterized in that a surface hardness of the layer is 0.05 to 20 GPa measured with a nano-indentation method.
Patent Document 3 does not disclose the following object. By stipulating the surface hardness measured with a nano-indentation method, a strain in the layers or in the interfaces produced during a laminating process of an organic light-emitting element may be removed, and a minute breakage or peeling off of the transparent electrode may be decreased to result in improvement of dark spot resistivity and lifetime of an element. Patent Document 3 does not refer to a problem such as breakage or peeling off of the electrode cause by bending when an organic light-emitting element is produced using a flexible transparent substrate.
Therefore, it is expected an achievement of an organic light-emitting element provided with an internal light extraction layer on a widely used flexible transparent substrate. The organic light-emitting element exhibits high light-emitting efficiency by light extraction, and it does not produce breakage of the electrode, or float or peeling off of the electrode when it is repeatedly bent.